Experimental determination of the Soret coefficient of ionic ferrofluids Influence of the volume fraction and the ionic strength

Forced Rayleigh Scattering (FRS) is a powerful technique for investigating heat and mass transfers in colloids. In the present work, we determine the Soret coefficient ST and the thermal diffusion coefficient DT of magnetic colloids (ferrofluids). It has been theoretically predicted that the thermal diffusion coefficient DT of colloids depends both on the particle/solvent interfacial interaction and on the interactions between the colloidal particles. In order to understand the microscopic behavior of the Soret effect in these ionic magnetic colloids, experiments are performed on aqueous samples of various volume fractions Φ and ionic strengths. The dominant effect on the Soret coefficient comes here from the particle/solvent interaction and determines its sign. Interparticle interactions have an influence on ST in the moderate concentration range where virial like expansions are possible. In this range (Φ ≤ 0.10) and within the experimental error bar, the thermal diffusion does not depend on the ionic strength of the dispersion and the Φ-dependence of the friction coefficient is comparable to that of hard spheres. At larger concentrations, the thermal diffusion drastically decreases as the colloid approaches its dynamical glass transition. KEYWORD: Ferrofluids, Forced Rayleigh Scattering, thermal diffusion, Soret effect, friction, dynamical arrest.